1. Field of the Invention
The invention relates to a friction unit, especially for synchronization devices of vehicle manual transmissions, which include two essentially annular friction elements, which, through relative motion, can be arranged to transmit torque through their contacting friction surfaces.
2. The Prior Art
Friction units of this kind are known and, for instance in couplings or brakes, or particularly in the aforementioned synchronization devices of gear boxes, enable a controlled friction-tight torque transmission. Thus, friction elements with essentially disk-shaped friction surfaces are known, but these present the drawback of a relatively high power requirement in order to press together the friction surfaces for the transmission of a specific torque. For this reason, especially in compactly built arrangements, frictional elements have become common in which the friction surfaces are conical, whereby, through appropriate selection of the cone angle, transmission of the required torque can be ensured even by means of a relatively minor coupling force.
In the case of the aforementioned manual transmissions, annular frictional elements are used as so-called synchronization rings for synchronization of the revolutions in the conical arrangement described. These frictional elements generally consist of a ring-shaped substrate or support made of brass or steel and a friction lining made of abrasive material, like for instance organic friction coatings or sintered friction coatings. During actual synchronization, the frictional element, having an interior and/or exterior friction surface, is pressed against one or two opposing conical surface(s), whereby the revolutions of the two gear shafts to be synchronized are brought into phase. With a manual transmission this is necessary during each shift from one gear to another.
Commensurate with the applicable state of the art, the designs of such frictional elements within the various manual transmissions differ very widely. However, with regard to their conical designs the friction surfaces show a commonality which, as mentioned, has proved advantageous due to space limitations in the gear boxes.
By analysis of a great number of friction units of the type mentioned that were inadequate with regard to a satisfactory synchronization effect, it was found that the required friction moments for adequate torque transmission often could no longer be attained after a relatively small number of shift or synchronization cycles, and up to now there has been no logical explanation for this.